This invention relates to a control system for a diesel engine having a combustion mode for low engine loads whereby fuel injection is carried out in the first half of the compression stroke.
Generally in diesel engines, fuel injection is carried out at around top dead center of the compression stroke and combustion is effected substantially simultaneously with fuel injection. This is known as a diffusion combustion. In practice, the injected fuel front mixes with air and forms a gas mixture, and thus the diffusion combustion and a slight premix combustion both occur. When the proportion of diffusion combustion is large, the emission of uncombusted fuel particles (particulate gas) increases, and when the proportion of premix combustion is large, the emission of No.sub.x gases increases. Consequently, it has been difficult to reduce No.sub.x and particulate gas emissions at the same time.
To reduce No.sub.x and particulate gas emissions simultaneously, various proposals have been made for combustion wherein fuel and air are mixed uniformly inside the cylinder before a compression ignition (uniform highly diffuse premix combustion). In the uniform highly diffuse premix combustion, the fuel injection timing is advanced to around bottom dead center of the compression stroke so that the fuel and air mix uniformly before compression ignition occurs. With the uniform highly diffuse premix combustion, because the fuel and air are mixed uniformly, it is possible to reduce particulate gas emissions, and because there are fewer local hot spots, it is also possible to reduce No.sub.x emissions.
However, because diesel engines use a compression ignition, and the ignition occurs as soon as the pressure inside the cylinder has risen sufficiently after the fuel is injected, with the uniform highly diffuse premix combustion, the ignition timing has not been stable and ignition timing control has been impossible. Consequently, in an engine set to a high compression ratio, there has been a risk of ignition occurring early, before compression top dead center is reached.
And when, in order to stabilize ignition, a low compression ratio is set, thermal efficiency drops and it becomes impossible to develop sufficient power for high speeds and high loads; fuel consumption deteriorates; and there is also a risk of a deterioration in cold-starting performance.